This invention is directed generally to electronic, high current power switches for switching electrical currents. The invention is more specifically directed to the manner that the switch, including its internal solid state switching devices and its internal connections, are electrically and thermally connected to each other, to high current electrical cables and to a heat sink. The invention minimizes the generation of heat and optimizes the conduction of heat to a heat sink from the cables, the switch terminals, the solid state switching devices and the high current internal connections.
The manufacturers of solid state, high current power switching devices have continuously been developing switching devices that are increasingly less resistive because they have been able to construct them with increasingly more transistors that are connected in parallel on a substrate. Consequently, as the technology of solid state switching devices has advanced, switching devices have been able to carry an increasing amount of electrical current.
Typically, a switch that uses solid state switching devices mounts the devices, as well as the switch terminals for connecting to cables, on a circuit board. The amount of heat that is generated in many switches and the removal of that heat to prevent a temperature rise to damaging levels is not a design factor that must be considered for small signal applications. However, heat is one of the most important design factors that must be considered for high current applications. When a switch must carry high electrical currents when switched to its ON state, heat is generated in the switch components, especially in its solid state switching devices, and must be conducted from the heat-generating components to a lower temperature heat sink. The sources of heat in the switch include the solid state switching devices and internal high current connections to the switching devices and to the switch terminals.
An important application of high current switches is for switching electrical power to a variety of power tools, motors and other heavy equipment and machines that are mounted to vehicles, such as ambulances, emergency vehicles, work trucks, mobility vehicles, recreational vehicles and buses. Because of the high power that this equipment requires, the switches are connected to the equipment through cables that must be considerably larger than common wires. The cables have cable terminals that are attached to their ends. The cable terminals are connected to switch terminals that are a component of the electrical switch and are conventionally mounted on a circuit board. Cables, like other components that carry electrical current, have maximum continuous current ratings. If they carry a current above that rating their temperature rises noticeably. The sources of heat in a cable are the resistance of the cable conductor, the resistance between the cable conductor and the cable terminals, the resistance of the cable terminals and the resistance of the interfaces between the cable terminals and the switch terminals. Cables can be made to handle greater current by making the cable conductor and cable terminal larger in cross section and by reducing the resistance of the interfacing components. But those improvement result in an increased cost of additional metal material and manufacturing costs for reducing interface resistance.
U.S. Pat. No. 7,522,405 to Fogleman discloses a high current switch with solid state switching devices that are sandwiched between two rectangular bar conductors. Each bar conductor is connected to a cable. The Fogleman switch uses the cables as a heat sink and therefore relies on heat conduction into the cables from its solid state switching devices, from its internal conductors and from its switch terminals. Consequently, the cables should be designed not only to dissipate the heat generated by the current through the cables themselves but also to additionally dissipate the heat conducted into the cables from the switch components, including its solid state switching devices, as well as heat generated at terminal interfaces.
With the Fogleman switch, the heat dissipation in the cables can be sufficient if the design includes cables that are oversized beyond the size required for dissipating the heat generated in the cables themselves. Use of the proper gauge cable is critical. However, if cables are undersized because they were chosen based only upon the maximum continuous current that the cables are expected to carry, the heat transfer from the Fogleman high current switch into the cables may be inadequate and result in excessive switch temperature and damage to the switch. Unfortunately, in order to reduce cost, it is not uncommon for designers of high current circuits, such as those in vehicle electrical systems, to select cables that are rated at a lower maximum continuous current than is experienced by the switch during its normal operation. Any continuous current through the cable that exceeds the cable's maximum continuous current rating raises the cable temperature. The increased temperature at least reduces heat flow from the Fogleman switch into the cables. A sufficient temperature increase will cause the cables to be a heat source and reverse the heat flow causing heat to be transferred from the cables into the solid state switching devices and result in damage. Furthermore, if the electrical circuit experiences an overcurrent beyond the maximum expected or designed current, the excessive temperatures and resulting damage would be even more likely.
It is therefore an object and feature of the invention to provide a high current electrical switch using solid state switching devices that not only reduces the heat generated in the switch components but also more efficiently and effectively transfers heat from the switch components to a heat sink.
A further object and feature of the invention is to provide a high current switch with structural features that increase the rate of heat removal so the switch can be operated at a maximum continuous current through the switch and its connected cables that is greater than the rated maximum continuous current of the switching devices and consequently, the switch can be constructed from less expensive solid state switching devices.
A further object and feature of the invention to provide a high current electrical switch that also transfers heat out of, instead of into, cables that are attached to the electrical switch.
A still further object and feature of the invention is to provide a high current electrical switch that has a heat transfer path directly to its heat sink from a high current electrical conductor that series connects two arrays of parallel connected solid state switching devices within the switch.
A further object and feature of the invention is to provide a high current electrical switch that can save energy by operating at a lower temperature and therefore at a lower total series resistance.